For liquid chromatography, it is necessary to produce a liquid stream which flows through the chromatographic column at high pressure with the greatest possible continuity. The pressures to be employed in this case are in the order of magnitude of several hundred Bars with a discharge capacity of up to 10 ml/min. With a pressure in the order of magnitude of several hundred Bars, an appreciable compression of the liquid occurs.
German OS No. 23 11 016 discloses a multiple piston pump of the aforementioned type, which comprises two piston-cylinder units, which are driven by a motor with the interposition of a transmission comprising gears constructed in an elliptical manner and arranged eccentrically so that the sum of the respective piston displacements of both cylinders in the compression direction is equal to a constant, so that the piston pump has a constant discharge capacity on account of its mechanical construction. The regulating system responding to the pressure of the liquid conveyed serves to vary the speed of the motor in the case of a charge in the pressure so that variations in the rate of delivery caused by the compression of the liquid are compensated for and the rate of delivery remains constant even with changes of pressure.
As mentioned, in the known piston pump, the constant discharge capacity depends on its mechanical construction. This construction must be carried out with extremely high accuracy, in order to ensure a constant discharge capacity in the case of small discharge capacities. Furthermore, the manufacture and assembly of elliptical and eccentric gears involves considerable difficulties.